Conventionally, isolated switching power supplies are known in which the state of the output side (secondary side) is transmitted as a feedback signal to a control circuit through an auxiliary winding of a transformer (For example, Japanese Patent Laid-Open No. 2002-136114).
FIG. 8 shows an isolated switching power supply including a conventional output voltage detection circuit. The switching power supply includes the output voltage detection circuit which detects a secondary side output voltage based on a voltage induced to an auxiliary winding.
The following will describe the switching power supply of FIG. 8.
A rectifier circuit 2 rectifies alternating voltage from an alternating current power supply 1. A smoothing capacitor 3 smoothes the rectified voltage from the rectifier circuit 2. The smoothed voltage is applied to a primary side winding 7-1 of a transformer 7.
A regulator 4 gradually charges a capacitor 5 based on the voltage from the smoothing capacitor 3 and operates so as to always keep the voltage across the capacitor 5 at a fixed value.
A control circuit 9 operated by the voltage across the capacitor 5 as the power supply voltage controls the on/off of a switching element 8 connected to the primary side winding 7-1, in a period during which the voltage across the capacitor 5 is within the range of a starting voltage to a stopping voltage which have been defined in the control circuit 9.
When the voltage across the capacitor 5 reaches the starting voltage and the control circuit 9 starts controlling the on/off of the switching element 8, power supply from the primary side winding 7-1 to a secondary side winding 7-2 and an auxiliary winding 7-3 is started. The switching power supply is a flyback power supply in which the primary side winding 7-1 and the secondary side winding 7-2 of the transformer 7 are opposite in polarity.
A capacitor 16 is a Y capacitor for removing noise.
A snubber circuit 6 clamps the peak voltage of the primary side winding 7-1. The snubber circuit 6 prevents a high spike voltage occurring when the switching element 8 is in a transition between on and off.
Voltage induced to the secondary side winding 7-2 is rectified and smoothed by a diode 11, capacitors 12 and 14, and a choke coil 13. As a result, direct-current power is supplied to a load 15. The voltage across the load 15 is the output voltage of the secondary side.
Since the auxiliary winding 7-3 of the transformer 7 has the same polarity as the secondary side winding 7-2, a voltage proportionate to the voltage induced to the secondary side winding 7-2 is induced to the auxiliary winding 7-3.
The capacitor 5 is connected to the auxiliary winding 7-3 via a diode 35, and the voltage across the capacitor 5 changes in proportion to the voltage induced to the auxiliary winding 7-3. For example, when the load 15 increases, the peak value of the voltage across the secondary side winding 7-2 decreases, and thus the peak value of the voltage across the auxiliary winding 7-3 and the voltage across the capacitor 5 also decrease. Conversely, when the load 15 decreases, the peak value of the voltage across the secondary side winding 7-2 increases, and thus the peak value of the voltage across the auxiliary winding 7-3 and the voltage across the capacitor 5 also increase.
For this reason, a conventional output voltage detection circuit 34 detects the voltage across the capacitor 5 as secondary side output voltage, and feeds back the voltage to the control circuit 9.
Referring to FIGS. 9A and 9B, the following will describe the operations of the conventional isolated switching power supply of FIG. 8 at a light load and a heavy load respectively. FIGS. 9A and 9B show voltage waveforms on each point of a light load and a heavy load, respectively, in the conventional isolated switching power supply of FIG. 8.
In FIGS. 9A and 9B, DRAIN represents a voltage on the high-potential side of the switching element 8, VA′ represents a voltage induced to the auxiliary winding 7-3, and VB′ represents a voltage obtained by rectifying and smoothing, through the diode 35 and the capacitor 5, the voltage having been induced to the auxiliary winding 7-3.
The conventional output voltage detection circuit 34 detects the voltage VB′ across the capacitor 5 as secondary side output voltage. Since the voltage VB′ corresponds to the peak value of the voltage VA′ induced to the auxiliary winding 7-3, as shown in FIGS. 9A and 9B, the conventional output voltage detection circuit 34 detects a higher voltage (the voltage having been clamped by the snubber circuit 6) than the original detection voltage.
Further, the conventional output voltage detection circuit 34 does not detect a voltage generated on the auxiliary winding 7-3 but detects the waveform of a voltage which is generated on the auxiliary winding 7-3 and reduced by the diode 35 and the capacitor 5. Therefore, the voltage detected by the output voltage detection circuit 34 is varied by the diode 35 and the capacitor 5.
Moreover, the voltage detected by the conventional output voltage detection circuit 34 is affected by the forward voltage variation components of the diode 11 on the secondary side.
When the forward voltage variations of the diode 11 on the secondary side are cancelled by the forward voltage variations of the diode 35 in the conventional isolated switching power supply, the isolated switching power supply can be used only for a low output power supply.
As described above, in the conventional output voltage detection circuit, the detection voltage is higher than the original detection voltage, causing a large detection error. Further, in the conventional output voltage detection circuit, the accuracy of detection considerably varies due to the variations of the diode 35 and the capacitor 5, which rectify and smooth the voltage having been induced to the auxiliary winding 7-3, and the forward voltage variation components of the diode 11 on the secondary side.
Therefore, the isolated switching power supply using the conventional output voltage detection circuit has a poor load regulation characteristic, which is a power supply characteristic. Moreover, the isolated switching power supply using the conventional output voltage detection circuit is difficult to be applied to a high output power supply.